This invention relates generally to a booster circuit for a universal serial bus (“USB”) and in particular to a circuit having a two stage buffer circuit coupled to the USB bus through a high pass resistor-capacitor filter.
Peripherals and external devices are commonly connected to a computer system using some type of bus arrangement. Traditionally, the connection was made using either a serial connection using a protocol such as RS-485 or via a parallel port using a protocol such as IEEE 1284. As the number of devices that needed to be connected increased, a new serial bus standard, known as universal serial bus (“USB”), was developed to aid in the interconnectability of computers and external devices. One aspect of the USB standard is that multiple devices could be connected to a single port on the computer using a device known as hub.
The USB standard provided a number of advantages over the previously used protocols. USB was designed to allow multiple external devices to be connected using a single standardized hardware interface. The standard also provided for power to be transferred to the devices allowing a number of devices to eliminate secondary power sources. Further, the devices could be connected and disconnected by the computer without rebooting or restarting the host computer. As a result of these features, the USB standard was widely adopted and the number of devices that coupled via USB grew rapidly.
To accommodate the increasing number of devices that connected via USB, peripherals known as USB “hubs” were developed that allowed multiple devices to be connected to a single USB port. Also, the USB standard increased the bit-rate that data could be transferred over the bus allowing devices such as external hard drives to become more practical. However, as the number of connected devices increases, the length of the bus also increases which in turn adversely impacts signal quality. Further, as the allowable bit-rate increased, the required level of signal integrity also increased.
Signal integrity is analyzed using an eye diagram that measures rise time, fall time, undershoot, overshoot and jitter. If a signal on the bus falls outside of the eye diagram specified by the USB standard, the signal will fail. A device that is normally USB compliant, meaning it transmits signals that comply with the standard, may start to fail if too many devices are coupled to the same bus, or if the physical length of the bus becomes too long. Thus, the number of devices and the speed of data transfer are limited by the quality of the signal.
While current USB bus topologies are sufficient for their intended purposes, it is desired to have computer systems incorporating longer USB bus lengths with more devices attached. In particular, it is desirable to have a system that is capable of boosting USB signs that have a slow edge rate to prevent rejection or signal delay.